In recent years, organic semiconductors comprised of amorphous film of organic substances are in wide use in a variety of electronic devices. For example, an organic amorphous flim is formed by preparing a coating composition comprised of a binder resin such as polycarbonate resin and a low molecular weight organic compound such as a triphenylamine derivative having photoelectric function dissolved in a suitable organic solvent and then by coating and drying the composition. The film thus formed is used as a positive hole transport layer in electrophotographic devices, as described in JP-A-1999-174707. Similarly, an organic amorphous flim is formed by preparing a coating dissolved in a suitable organic solvent and then by coating and drying the composition. The flim in solar batteries, as described in JP-A-2001-007355.
As described above, organic semiconductor films comprised of organic amorphous film have been prepared by preparing a coating composition using a low molecular weight organic compound having photoelectric function together with a binder resin and then coating the composition on a suitable substrate and drying the composition. However, many of the low molecular weight organic compounds that have hitherto been known have low oxidation potentials, and accordingly when they are formed to organic semiconductor films by a coating method, they are easily oxidized, so that it is not easy to form a film using such low molecular weight organic compounds. They have also no sufficient reversibility in oxidation-reduction process so that it is difficult to prepare organic semiconductor film durable and suitable for practical use. In addition, the resulting organic semiconductor films have no sufficient heat resistance and hence the electronic devices using such organic semiconductor films are inferior in stability and durability.
As typical low molecular weight organic compounds that have photoelectric function and are usable for preparing organic semiconductor films, there have been known such compounds as N,N,N′,N′-tetramethylbenzidine, N,N,N′,N′-tetraphenyl-(1,1′-biphenyl)-4,4′-diamine, N,N′-diethyl-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, or N,N,N′,N′-tetra(3-methylphenyl)-4,4′-diaminostilbene. However, these low molecular weight organic compounds form only amorphous films that are by themselves not so stable as to be used as electric charge transport agents in organic photosensitive elements. Accordingly, they are dispersed in a binder resin (that is, diluted with a binder resin), and the resulting dispersion is applied to a substrate to form an amorphous film.
Thus, the known low molecular weight organic compounds that form an organic amorphous film are diluted with a binder resin and are influenced by the binder resin which forms a matrix for the amorphous film so that the organic amorphous film cannot exhibit sufficiently the properties that they originally possess. In addition, if the known low molecular weight organic compounds form an amorphous film that is relatively stable at normal temperatures with the aid of a binder, they have low glass transition temperatures so that the film is poor in heat resistance and is not suitable for practical use.
Accordingly, the development of low molecular weight organic compounds that have photoelectric conversion function and are capable of forming amorphous film by themselves at normal temperatures or higher has been pushed on with in recent years, and as results, some nitrogen-containing polynuclear aromatic compounds called star-burst molecules have been proposed as such low molecular weight organic compounds.
The star-burst molecules are divided into three groups based on their molecular structures: compounds having triphenylamine structure (triphenylamines), compounds having triaminobenzene structure (triaminobenzenes) and compounds having triphenylbenzene structure (triphenylbenzenes). Beside the above-mentioned, compounds having triphenylmethane structure are also proposed.
The triphenylamines include, for example, 4,4′,4″-tris-(N,N-diphenylamino)triphenylamine (TDATA) (1) having the structure
as described in JP-A-1990-224353; 4,4′,4″-tris(N-phenyl-N-m-tolylamino)triphenylamine (m-MTDATA) (2) having the structure
as described in JP-A-1990-224353; 4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine (2-TNATA) (3) having the structure
as described in JP-A-1996-291115; and 4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino)triphenylamine (1-TNATA).
These triphenylamines are reversible in oxidation-reduction process, however, they have low oxidation potentials (oxidation potential against Ag/Ag+ electrode, the same hereunder) of about 0.1V or less so that there is a problem in that they are easily oxidized when they are formed to organic semiconductor film by a coating method.
m-MTDATA has a glass transition temperature of about 77° C. so that it is difficult to use the compound in practical electronic devices, and on the other hand, 2- or 1-TNATA has a glass transition temperature of about 110° C. and is capable of forming heat-resistant organic amorphous film, but the compound is readily crystallized so that the resulting organic amorphous film is lacking in stability or durability.
The triphenylbenzenes include, for example, 1,3,5-tris(4-(N,N-diphenylaminophenyl)benzene (TDAPB) having the structure (4)
and 1,3,5-tris(4-(N-tolyl-N-phenylaminophenyl)benzene (MTDAPB) (5) having the structure
as described in Bando Technical Report, Vol. 2, pp. 9–18, 1998 (Bando Chemical Industries, Ltd.).
The triphenylbenzenes are capable of forming amorphous film and have oxidation potentials in the range of 0.6–0.7V, but they are irreversible in oxidation-reduction process so that they are not suitable for use in practical use as organic semiconductors.
In turn, the triaminobenzenes include, for example, 1,3,5-tris(N-methylphenyl-N-phenylamino)benzene (MTDAB) having the structure (6)
as described in Bando Technical Report, Vol. 2, pp. 9–18, 1998 (Bando Chemical Industries, Ltd.). The triaminobenzenes also have oxidation potentials in the range of 0.5–0.6V, but they are irreversible in oxidation-reduction process, like the above-mentioned triphenylbenzenes, and in addition, they have glass transition temperatures as low as about 60° C. or less so that they are not suitable for use in practical use as organic semiconductors. They have further problems in heat resistance.
The invention has been accomplished to solve the above-mentioned problems in the star-burst molecules having triaminobenzene structure for use as materials for organic semiconductors.
Accordingly, it is an object of the invention to provide novel 1,3,5-tris(arylamino)benzenes that have oxidation potentials in the range of about 0.5–0.6V and high glass transition temperatures and that are superior in reversibility in oxidation-reduction process and heat resistance so that they are readily formed to organic semiconductor film by a coating method or vacuum deposition method, as well as they are capable of forming stable and durable high-performance organic semiconductor film by themselves because they are capable of forming amorphous film by themselves with no aid of binder resins at normal temperatures or higher.